High boiling silicon compounds, such as compounds having Si—Si linkages, Si—O—Si linkages, or Si—Ca—Si linkages (where subscript a is 1 or more), in their molecules are formed as the undesirable by-products of industrial processes that produce monosilanes as chemical intermediates. For purposes of this application, the term ‘monosilane’ refers to a silane species having one silicon atom with four substituents bonded thereto. Monosilanes include, but are not limited to, trichlorosilane (HSiCl3), silicon tetrachloride (SiCl4), dimethyldichlorosilane ((CH3)2SiCl2), dimethylhydrogenchlorosilane ((CH3)2HSiCl), methylhydrogendichlorosilane (CH3HSiCl2), and methyltrichlorosilane (CH3SiCl3). The term ‘high boiling polymers’ refers to compounds having more than one silicon atom, and they are exemplified by the high boiling silicon compounds described above. The high boiling polymers have boiling points above the boiling points of the chloromonosilanes, e.g., 70° C. and above, alternatively 80° C. and above. High boiling polymers are present in residues produced as waste streams from industrial processes for making chloromonosilanes and methylchloromonosilanes (e.g., the direct process). High boiling polymers are also present in residues produced in industrial processes for making silicon (e.g., processes for making solar grade silicon and/or semiconductor grade polycrystalline silicon). The term ‘residue’ refers to any stream containing the high boiling polymers.
In an industrial process for making chloromonosilanes, hydrogen chloride (HCl) is reacted with metallurgical grade silicon (Si) in an uncatalyzed reaction system to produce trichlorosilane (HSiCl3). Another process involves reaction of methyl chloride with metallurgical grade Si in a system catalyzed with copper compounds and promoted with a number of various metal additives such as zinc, tin and phosphorous to form methylchloromonosilanes (MCS). A portion of the silicon and chloride (in the form of HCl from the HSiCl3 process or CH3Cl from the MCS process) in the initial reaction and in downstream processes is lost to the formation of by-product high boiling polymers.
The HSiCl3 process residue may comprise disilanes of the formula HbSi2Cl(6-b), where subscript b has a value ranging from 0 to 6, alternatively 0 to 4; and disiloxanes of formula HcSi2OCl(6-c), where subscript c has a value ranging from 0 to 6. In the HSiCl3 process, these high boiling polymers include tetrachlorodisiloxane (HCl2SiOSiCl2H, H2Si2OCl4), pentachlorodisiloxane (HCl2SiOSiCl3, HSi2OCl5), hexachlorodisiloxane (Cl3SiOSiCl3, Si2OCl6), hexachlorodisilane (Si2Cl6), pentachlorodisilane (HSi2Cl5), tetrachlorodisilane (H2Si2Cl4), and trichlorodisilane (H3Si2Cl3).
The MCS process residue may comprise disilanes of the formula MedSi2Cl(6-d), where subscript d has a value ranging from 0 to 6 and disiloxanes and/or silalkanediyl compounds of formula MeeSi2XCl(6-e), where subscript e has a value ranging from 0 to 6, and X is an oxygen atom or an divalent hydrocarbon group group. In the MCS process, these high boiling polymers include Si2Cl6, tetramethyldichlorodisilane (Me4Si2Cl2), trimethyltrichlorodisilane (Me3Si2Cl3), tetramethyltetrachlorotrisilane (Me4Si3Cl4), tetramethyldichlorodisilmethylene (Me2ClSiCH2SiMe2Cl), trimethyltrichlorodisilmethylene (Me2ClSiCH2SiMeCl2), trimethyltrichlorodisilethylene (Me2ClSi(CH2)2SiMeCl2), trimethyltrichlorodisilpropylene (Me2ClSi(CH2)3SiMeCl2), Me2ClSiCH2Si(Me)(Cl)SiMeCl2, Me2ClSiCH2Si(Me)(Cl)CH2SiMeCl2, and trimethyltrichlorodisiloxane (Me3Si2OCl3); where Me represents a methyl group.
The high boiling polymers, such as the disilanes, from the HSiCl3 and MCS processes can be recovered and converted to useful monosilanes via hydrogenation, chlorination or hydrochlorination, but catalysts may be required to make the reactions economical. In the MCS system, the most economically favorable system is the hydrogenation of methylchlorodisilanes with the use of in-situ catalysts in the process residue, which contains the high boiling polymers. MCS process residue can be prepared for downstream processing so it is rich with in-situ catalysts valuable for hydrogenation. In the HSiCl3 process, the HSiCl3 process residue containing the high boiling polymers is typically disposed of by quenching and/or incineration.